It is well known that, as a general rule, as the diameter of carbon fibers is decreased, strength generally increases. The reasons for this are usually ascribed to improved molecular orientation (e.g., increased graphitic structure) and to a reduction in the number of flaws due to the improved quality of the cross-sectional filament structure. At the extreme of the continuum lie carbon nanotubes, which ideally are fully graphitic without flaws in the structure of the walls. However, the realization of the potential of the mechanical benefits of these materials is hindered by the requirement of having to transfer load along the fiber length between fibers via mechanical entanglements caused by frictional and van der Waal's interactions between the carbon nanotubes themselves and between adjacent fibers through shear coupling such as from a matrix resin.
Currently, continuous carbon fibers with nanoscale features are not available except on the research level. Most carbon fibers with nanoscale features are either carbon nanotubes or carbon nanofibers. Carbon nanofibers are generally vapor-grown or electrospun. Vapor-grown carbon fibers typically comprise a range of lengths and are not continuous. By contrast, electrospun carbon fibers can be made continuously. However, there are many shortcomings to electrospinning.
In electrospinning an electric field is generated between an oppositely charged polymer fluid and a fiber-collection ground plate. A polymer solution is added to a glass syringe with a capillary tip. As the electrical potential is increased, the charged polymer solution is attracted to the screen. Once the voltage reaches a critical value, the charge overcomes the surface tension of the polymer cone formed on the capillary tip of the syringe and a jet of ultrafine fibers is produced. As the charged fibers are splayed, the solvent quickly evaporates and the solidified fibers are accumulated randomly on the surface of the collection screen. This results in a nonwoven mesh of nano to micron scale fibers. Varying charge density, polymer solution concentration and the duration of electrospinning can control fiber diameter and mesh thickness.
The first problem with electrospinning is related to the difficulty in collecting and collimating the fibers in order to handle them as ordered fibers. Currently, it is only possible on length scales of several inches to one foot. Second, and perhaps more important, electrospinning heads may not be placed in too close of proximity to each other as the electric fields emanating from each head can interfere with the other. Due to this limitation, in order to produce a large number of fiber ends, a commercially impractical area would be required to accommodate these on a production floor. Customarily, a large number of fiber ends is needed because, in order to approximate the typical 3-24000 filament ends (5-10 microns in diameter) present in commercial carbon tows in cross-sectional area, somewhere between 1000 and 10,000 times as many ends would be needed, thereby necessitating several million electrospinning heads to achieve this goal. For large-volume production of continuous fibers, this becomes untenable.
As such, there remains a need for continuous carbon fibers with nanoscale features. More specifically, there remains a need for a continuous, hollow carbon fibers with nanoscale features. Additionally, there remains a need for a method of producing a continuous, hollow carbon fiber with nanoscale features. More specifically there remains a need for a cost-effective method of producing a large-volume of continuous, hollow carbon fibers with nanoscale features such that their use in composites-based, primary load-bearing structures such as for aircraft is practical. Moreover, there remains a need for both melt-spinnable and solution-spinnable methods for producing continuous, hollow carbon fibers with nanoscale features.